The growing demand for wireless communications is constantly increasing the requirement for enhancing the capacity of service available to the users. One of the solutions known in the art to increase the capacity of a given wireless network is the reuse of available frequencies. An example of such method of operation is the Reuse-1 by which the same range of frequencies is used at each of the system's cells (as opposed to a solution whereby the same range of frequencies is used per a number of cells in order to avoid overlapping between adjacent cells). The advantage of using the Reuse-1 method is that the frequency range used in each of the cells can be considerably wider than that used in other types of reuse methods, while the obvious problem is how to prevent interferences between the adjacent cells.
One way of handling the overlapping problem is by using symbol repetition (i.e. transmitting each symbol more than once, say two-three times). All symbols together with their replications are received and stored at the receiving end and the decision on what should have been received is more robust as the addition of the data signals will be coherent while addition of the noise bits will not have that property.
Still, this solution does not solve another problem associated with this method, namely the channel estimation. Accurate channel estimation is required for demodulation and in particular for effective combining of the low signal to interference and noise ratio (hereinafter “SINR”) repeated symbols. The procedure of carrying out channel estimation is based upon pilot symbols being transmitted along with the data symbols and received at the receiving end, but the problem here is that the pilot symbols interfere with adjacent cell's pilot symbols. In CDMA type of communication systems (e.g. IS-95) this problem is solved by increasing the pilot power and using pseudorandom spreading of the pilot signals. However, in OFDMA type of communication systems this type of solution is not applicable to solve the problem, since pilot tones in the desired cell and in the interfering cells coincide. Increasing the pilot power causes increase also in the interference level from the pilots in adjacent cells, leaving the pilot SINR unchanged. For this reason, channel estimation algorithms assuming that the signal is accompanied by regular thermal noise will have a highly degraded performance.
EP 1,276,251 discloses a method for calculating a weighting vector for an antenna array whereby in order to increase the carrier to interference+noise ratio (“CINR”) a method is provided to calculate a weighing vector to support interference cancellation in an Orthogonal Frequency Division Multiplexing (OFDM) multi-carrier modulation technique. By the solution described in this publication, when there are two or more interferences with known interference channel vectors, the antenna weighing vectors is selected from among the orthogonal complement of all interference channel vectors, after carrying out orthogonalization procedure upon the respective interference channel vectors.
The above reference mentioned is hereby incorporated by reference as if fully set forth herein.